1. Field of the Invention
The present invention relates to an optical scanning device that scans a predetermined object with a laser light and an image forming apparatus including the optical scanning device.
2. Description of the Related Art
An optical scanning device that outputs a laser light and scans a predetermined object with the laser light is used as an exposure device in an image forming apparatus such as a copier, a printer, and a fax machine. An exposure device scans a photosensitive drum with a laser light based on image information from the outside and forms an electrostatic latent image on the photosensitive drum.
As illustrated in FIG. 13, the exposure device includes, as basic components, a laser light source 81 that outputs a laser light, a polygon mirror 82 that deflects the laser light and scans a photosensitive drum with the laser light, a polygon motor (not shown) that rotates the polygon mirror 82, a control board 83 that controls the polygon motor, an fθ lens 84 and a reflective mirror 85 that guide the laser light deflected by the polygon mirror 82 to the photosensitive drum, and a housing 86 that houses the aforementioned optical system elements.
Recent image forming apparatuses demand faster development operations. Achieving such an objective requires rotating the polygon mirror 82 of the exposure device at high speed. Therefore, a temperature of the control board 83 that controls the polygon motor tends to rise.
In addition, in keeping with recent demands for smaller image forming apparatuses, exposure devices must also be downsized. While the polygon mirror 82 and the control board 83 are arranged in an arrangement space S1 defined by an inner wall surface 86a that is a part of a wall part 87 of the housing 86, the arrangement space S1 must be minimized in order to meet the needs for downsizing. In other words, a distance between the polygon mirror 82 and the wall part 87 must be minimized. Furthermore, in order to fulfill the need for downsizing, optical system elements such as the polygon mirror 82 and the fθ lens 84 are arranged as close to each other as possible.
Therefore, heat generated at the control board 83 is more likely to be retained in the arrangement space S1 to cause a rise in the temperature of the arrangement space S1, while temperatures of spaces in the housing 86 other than the arrangement space S1 drop in comparison. Although an airflow is generated by a rotation of the polygon mirror 82, the airflow is blocked by the inner wall surface 86a that defines the arrangement space S1 or by the fθ lens 84 and is retained in the arrangement space S1, thereby preventing heat generated at the control board 83 from being guided to the outside of the arrangement space S1.
As described above, since an uneven temperature distribution occurs inside the housing 86, there is a risk of a nonuniform thermal deformation occurring between a high temperature area and a low temperature area of the housing 86. A nonuniform thermal deformation of the housing 86 causes displacement of arrangement positions of optical system elements and displacement of relative positions among the optical system elements. For example, if the fθ lens 84 is displaced from a predetermined arrangement position at both longitudinal ends, a laser scan line may become bent or a so-called left-right magnification difference may occur. As a result, it becomes difficult to form a favorable toner image on the photosensitive drum.
A first prior art is known as an example of a technique for suppressing a nonuniform thermal deformation of a housing. An optical scanning device according to the first prior art includes a housing comprising an outer wall, an inner wall, a bottom wall, and an upper cover, wherein a flow channel defined by the outer wall, the inner wall, the bottom wall, and the upper cover is formed on a outer circumferential side of the housing. In addition, a polygon mirror and a drive unit thereof are arranged inside the flow channel.
With the optical scanning device according to the first prior art, since an airflow generated by a rotation of the polygon mirror endlessly circulates inside the flow channel along an outer circumference of the housing, heat generated by the drive unit is not retained in one spot and is instead carried by the airflow to be cooled within the flow channel. Accordingly, the generation of an uneven temperature distribution in the housing and, in turn, the occurrence of a nonuniform thermal deformation of the housing is suppressed.
However, with the optical scanning device according to the first prior art, since the flow channel is provided on the outer circumferential side of the housing, a size of the housing increases by just that much. As a result, recent demands for smaller image forming apparatuses cannot be fulfilled.